1. Field of the Invention
The present invention relates to a balanced-unbalanced converting circuit, a balanced-unbalanced converter, which are for operation in a high frequency band, and a communication device including the same.
2. Description of the Related Art
The Marchand balun circuit shown in FIG. 9 is a known wideband balanced-unbalanced converting circuit. In FIG. 9, transmission lines 5a, 5b, 6a, and 6b each having a quarter-wavelength at an operating frequency are shown. One end of each of the transmission lines 6a and 6b is grounded, and the other ends thereof are signal input-output ports 2 and 3, respectively. One end of the transmission line 5b is open, and the other end thereof is connected to one end of the transmission line 5a. The other end of the transmission line 5a is a signal input-output port 1.
With this configuration, the transmission lines 5a and 5b and the transmission lines 6a and 6b are coupled via electromagnetic fields, respectively, so that a phase difference of 180xc2x0 is produced between the open end of the transmission line 5b and the signal input-output port 1 of the transmission line 5a. Accordingly, this circuit functions as a balun in which the ports 2 and 3 act as balanced ports, and the port 1 acts as an unbalanced port.
U.S. Pat. 5,880,646 discloses a balanced-unbalanced converter including coaxial transmission lines. In the balanced-unbalanced converter, two quarter-wavelength transmission lines are provided in a dielectric block. A transmission line is formed on the outer surface of the dielectric block so as to connect first ends of the respective two transmission lines to each other. The second ends of the two transmission lines are balanced ports, and an unbalanced port is defined between the second end of one of the two transmission lines and ground.
In a conventional Marchand balun circuit as shown in FIG. 9, generally, the transmission lines 5a, 5b, 6a, and 6b are formed on a dielectric substrate. Therefore, the Q value of the transmission lines is low, and in some cases, unnecessary radiation becomes a problem. Furthermore, in a balanced-unbalanced converting circuit containing coaxial transmission lines as disclosed in the above-mentioned U.S. Pat. 5,880,646, the transmission line extends a distance of a half-wavelength from one of the balanced ports. Accordingly, a loss caused by this transmission line deteriorates the balance characteristic (the difference between the amplitudes at the balanced ports).
Further, if the Marchand balun circuit shown in FIG. 9 is formed by use of a dielectric coaxial line, it is necessary to provide a transmission line with a length of a half-wavelength (total of the transmission lines 5a and 5b) and transmission lines 6a and 6b in parallel to the half-wavelength transmission line in a dielectric block. This causes the interval between the open ends of the quarter-wavelength transmission lines 6a and 6b to be excessively short. Thus, from a structural standpoint, it becomes difficult to form the balanced input-output ports 2 and 3.
In the conventional Marchand balun circuit, one unbalanced signal is converted to one balanced signal, or vice versa. That is, the conventional Marchand balun circuit is not capable of demultiplexing one balanced signal into two unbalanced signals, nor of multiplexing two unbalanced signals to provide as one balanced signal.
Accordingly, the present invention provides a balanced-unbalanced converting circuit, a balanced-unbalanced converter, which are effective in solving problems caused by an excessively short interval between the above-mentioned balanced ports, and can be effectively operated e.g., in a frequency band which is higher than the quasi-microwave band, and a communication device including the same.
The present invention further provides a balanced-unbalanced converting circuit and a balanced-unbalanced converter, which are formed by use of coaxial transmission lines, respectively, in which the loss caused by the transmission lines is reduced, and deterioration of the balance characteristic is prevented, and a communication device including the same.
The present invention also provides a balanced-unbalanced converting circuit and a balanced-unbalanced converter, each of which is able to multiplex two unbalanced signals with different frequencies, which are output from, e.g., two voltage control oscillators, and wherein the outputs are mixed, that is, the two unbalanced signals are multiplexed and thereby converted to one balanced signal, and a communication device including the same.
To achieve these objects, according to a first aspect of the present invention, there is provided a balanced-unbalanced converting circuit which comprises a first transmission line having both ends open, and a second transmission line having both ends grounded, arranged substantially in parallel to the first transmission line, and having an electrical length substantially equal to the electrical length of the first transmission line, the first transmission line having balanced ports connected to both of the ends thereof, the second transmission line having an unbalanced port connected substantially to the center thereof.
As described above, by connecting the balanced ports to the two ends of the first transmission line, respectively, the interval between the balanced ports is wide, so that the balanced ports can be easily formed. Moreover, unnecessary coupling between the balanced ports can be reduced, and an excellent balance characteristic can be obtained.
Preferably, in a second aspect of the invention, the balanced-unbalanced converting circuit comprises a first transmission line having both ends open, and second and third transmission lines arranged substantially in parallel to the first transmission line, the third transmission line having an electrical length substantially equal to that of the first transmission line and different from that of the second transmission line, and having both ends grounded, the first transmission line having balanced ports connected to both of the ends thereof, the second and third transmission lines each having an unbalanced port connected substantially to the center thereof. Thereby, a balanced-unbalanced converting circuit provided with one balanced port and two unbalanced ports, corresponding to two frequencies, can be obtained. That is, the balanced-unbalanced converting circuit can multiplex or demultiplex a signal, in addition to the balanced-unbalanced signal converting function.
Also preferably, in a third aspect of the invention, the electrical length of the first transmission line is in the range between the electrical lengths of the second and third transmission lines. By reducing the difference between the electrical lengths of the first and second transmission lines, and the difference between the electrical lengths of the first and third transmission lines, respectively, a good balanced-unbalanced conversion characteristic can be obtained with respect to two frequency bands.
Preferably, in the balanced-unbalanced converter, the first and second transmission lines in the above-described balanced-unbalanced converting circuit each comprise a microstrip line or strip line produced by forming a conductor film on a dielectric substrate. Thereby, the balanced-unbalanced converter can be easily formed on the dielectric substrate and the balanced-unbalanced converter can be easily connected to another high frequency circuit to be formed on the dielectric substrate.
Also preferably, in the balanced-unbalanced converter, the first and second transmission lines in the above-described balanced-unbalanced converting circuit each comprise a dielectric coaxial transmission line produced by forming a conductor film in a dielectric block. Thereby, a small-sized balanced-unbalanced converter having a low loss and a low unnecessary radiation characteristic can be formed.
Furthermore, at least a part of one or more of the conductor films may be a thin film lamination electrode having an area in which plural thin film conductor layers and plural thin film dielectric layers, each having a thickness smaller than the skin depth at an operating frequency are alternately laminated. Thereby, a low loss can be attained.
Furthermore, according to the present invention, there is provided a communication device which comprises the above-described balanced-unbalanced converter provided, e.g., in a high frequency circuit section. Thereby, a communication device reduced in size and having high efficiency can be provided.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.